Next generation mobile communication systems in which video communication and data communication are mainly performed require capability far higher than that for the third generation mobile communication systems (IMT-2000), so that it is necessary to sufficiently achieve large capacity, speed-up and broadband and the like. Thus, various communication environments in doors and out of doors can be considered. In data transmission in a downlink direction, not only the unicast scheme but also multicast scheme and broadcast scheme are performed. Especially, it has become increasingly valued in recent years to transmit the multimedia broadcast and multicast service (MBMS) channel. The MBMS channel may include multimedia information that is broadcasted to many specific or non-specific users, and may include various contents such as voice, characters, still images and moving images and the like (refer to non-patent document 1 as to the trend of future communication systems).
On the other hand, in broadband mobile communication systems, effects of frequency selectivity fading due to multipath environment become remarkable. Therefore, the orthogonal frequency division multiplexing scheme (OFDM) is highly expected as the next generation communication scheme. In the OFDM scheme, one symbol is formed by adding a guard interval part to an effective symbol part including information to be transmitted, and a plurality of symbols are transmitted during a predetermined transmission time interval (TTI). The guard interval part is generated using a part of information included in the effective symbol part. The guard interval part is also called cyclic prefix (CP) or an overhead.
On the other hand, the MBMS channel is different from the unicast channel in that the MBMS channel including same content is transmitted from a plurality of cells. The unicast channel is transmitted from one cell to a specific user as a general rule. The “area 1” shown in FIG. 1 includes three base stations BS1, BS2 and BS3, and a same MBMS channel is transmitted in this area. Such an area may be called a MBMS area. Similarly, “area 2” includes three base stations BS11, BS12 and BS13, and a same MBMS channel is transmitted in this area. In general, the MBMS channel transmitted in the area 1 is different from that transmitted in area 2, but, they may be the same intentionally or coincidentally. A mobile terminal (although it may be a communication terminal including a mobile terminal and a fixed terminal more generally, the mobile terminal is taken as an example for explanation for the sake of simplicity of explanation) receives MBMS channels of the same content transmitted from a plurality of cells. The received MBMS channels form many coming waves or paths according to the length of the radio propagation routes. Based on the property of the symbol of the OFDM scheme, when delay difference of the coming waves falls within the range of the guard interval, these plurality of coming waves can be combined (soft combining) without inter-symbol interference, so that reception quality can be improved because of the path diversity effect. Therefore, the guard interval length for the MBMS channel is set to be longer than the guard interval length for the unicast channel.
By the way, when a unicast channel is transmitted to a mobile terminal, each of a pilot channel, a control channel and the unicast channel is multiplied by a cell-specific scrambling code that is specific to a particular cell, and they are transmitted. The mobile terminal performs channel estimation and other processes based on the received pilot channel, performs channel compensation on the control channel and the unicast channel, and performs demodulation process after that. Since the scrambling code is different for each cell, a desired signal can be distinguished from an interference signal from other cells. However, if the unicast channel is simply replaced with the MBMS channel in this scheme (if a scrambling code that is different for each cell is used for transmission of the MBMS channel), the mobile terminal needs to distinguish signals (more particularly, pilot channels) from surrounding base stations for processing, but, it is difficult. From this viewpoint, it is proposed to prepare a scrambling code (common scrambling code) common to a plurality of cells included in a MBMS area separately for MBMS.
FIG. 2 schematically shows a manner when using the common scrambling code when transmitting the MBMS channel. In the example shown in the figure, a unicast channel is transmitted in a subframe (a unit transmission frame including a predetermined number of continuous symbols) in the first cell 1, and a MBMS channel is transmitted in another subframe. Also in the second cell 2, a unicast channel and a MBMS channel are transmitted. In this case, different scrambling codes are used between the cell 1 and the cell 2 as for the unicast channel. The scrambling code is different for each symbol. As to the MBMS channel, a same scrambling code (common scrambling code) is used in the cell 1 and cell 2. The common scrambling code is also different for each symbol.
As mentioned above, by separately preparing scrambling codes for each of the unicast channel and the MBMS channel, inter-cell interference of the MBMS channel may be directly avoided. However, if this method is adopted, different scrambling codes are applied to a pilot channel associated with the unicast channel and a pilot channel associated with the MBMS channel. Therefore, the pilot channel associated with the MBMS channel cannot be used for channel estimation on the unicast channel. Therefore, this method is not desirable from the viewpoint for achieving effective use of resources. When the unicast channel and the MBMS channel are time-multiplexed, if the duration of the MBMS channel becomes long, such inconvenience may become more and more serious.
As to this point, the technique of the non-patent document 2 proposes to use the cell-specific scrambling code also for the MBMS channel similarly to the unicast channel as shown in FIG. 3 without introducing the common scrambling code. In the technique, the scrambling code applied for the MBMS channel is common to a plurality of symbols in each subcarrier. That is, a same scrambling code is used for a same frequency component. For example, in the first cell 1, a same scrambling code A1 is applied for a pilot channel and a MBMS channel mapped to a frequency component f, and in the second cell 2, a scrambling code A2 that is different from A1 is applied for a pilot channel and a MBMS channel mapped to a frequency component f. When these are transmitted from the base station, and are received by a mobile terminal, all data (pilot channel and MBMS channel) having the frequency component f are multiplied by the same scrambling code (A1+A2). Therefore, as to the frequency component f, the mobile terminal can combine them in phase, so that the mobile terminal can combine them while performing soft combining for the MBMS channel.    [Non-patent document 1] Ootsu: “Challenge to Systems beyond IMT-200—approach from wireless—”, ITU journal, Vol. 33, No. 3, pp. 26-30, March 2003    [Non-patent document 2] R1-060182, 3GPP TSG RAN WG1 AdHoc Meeting, Helsinki, Finland, 23-25 Jan., 2006